This invention relates to a magnetic means and circuits of improved efficiency and, in particular, to a magnetic circuit for reducing leakage flux conducted through magnetically permeable material providing a curved or arcuate surface to which the flux may be directed.
In all magnetic circuits which include sources of magnetomotive forces (mmf), e.g., electromagnets and permanent magnets, it is recognized that a certain leakage flux will occur. Leakage flux may take several forms and is dependent on many factors, including the geometry of a magnetically conductive member (as, for example, portions of a motor stator), the influence of magnetic fields external to the field of interest, the strength of the magnetic field, and the proximity of magnetically-conducting materials to the field. In the case of motors, flux loss is commonly encountered at the sides of the poles and also in the vicinity of the stator winding or permanent magnet. In the motor, it is desirable, of course, to transfer as nearly as possible all the flux from the magnetic source across the face of the pole, the air gap, the rotor and thence to the opposite pole or flux return path. Any flux which does not follow that path, i.e. leakage flux, is not available to interact with the current in the armature and, therefore, the available torque of the motor is directly related to the efficiency of flux transfer between the source and the air gap. The same considerations apply to other electromagnetic equipment such as moving armature relays and electrochemical actuators and transducers.
In order to obtain better flux density in the magnetic circuits of motors and other electromagnetic equipment, certain expedients have been employed. For example, since the poles and pole pieces of conventional motors having larger numbers of poles generally taper radially inwardly, the areas of the pole faces adjacent the rotor are considerably smaller than the radially outward surfaces of the pole pieces, so that the flux density increases nearer the rotor. By and large, flux leakage from the pole pieces has been compensated by increasing the strength of the magnetic field or by magnetically shielding the poles from any closely spaced magnetically permeable material which otherwise would tend to aggravate the leakage problem.
Magnetic shielding involves the addition of low permeability parts, which consume space, and is not effective, in itself, to allow generation of unusually high magnetic flux densities. Yet other expedients have been to change the geometry of the pole piece so that its external surfaces are shaped so as not to be conducive to the establishment of intense leakage-inducing mmf field concentrations.
In one known stator assembly for a two-pole electric motor, improved magnetic flux density at the air gap could be obtained by mounting a pair of high-strength magnets on two obtusely angled, flat surfaces of the pole piece near the stator periphery, although the purpose of this particular construction apparently is simply to obtain more flux lines at the large face of a radially thin pole piece. The polar axes of these magnets intersect the axis of rotation of the motor so that the magnets, in that respect, bear a conventional relationship to the motor structure. That is, as is conventional in this type of motor, the polar axes of all independent magnetic sources (e.g., field coils) are generally located on radii through the axis of rotor rotation. In this particular assembly, however, the structure is somewhat different in that two independent mmf sources share a common pole piece. There is thus a tendency of the magnetic flux lines to realign along some median axis of the pole piece. However, a substantial amount of leakage occurs at sides of the pole piece which are not shielded in any way. Furthermore, because the angle is large between the surfaces of the pole piece covered by the permanent field magnets, very little surface area convergence is present and it is therefore difficult to obtain intensification of the flux density at the working air gap.
Thus, although motors of the foregoing design could be effective in improving the magnetic flux transfer from the mmf source to the rotor, substantial flux leakage still occurs, flux density amplification at the working air gap is severely restricted and the design requires a considerable amount of peripheral surface area upon which to mount the mmf field sources. Oftentimes, the motor geometry does not allow for the bulkiness of this arrangement.
Even in those prior art devices having an acceptable efficiency of magnetic transfer in the magnetic stator circuit, there nevertheless is a limit on the absolute field strength that can be achieved, owing to the degree of flux loss from the magnetically conducting pole element. Another type of motor offering potentially improved efficiency is disclosed in German Patent No. 1,174,418 (1964) wherein each of the three sides of a pole piece converging toward an air gap is covered by a magnetic source. Each magnetic source, if properly selected, can resist and oppose leaking flux from the other sources. In the present invention, flux loss is diminished and, as will appear later, the magnetic field density can be increased to values exceeding the flux density of the usual single mmf source to achieve flux densities in excess of 20,000 gauss. Further, the invention gives an arrangement by which the efficiency of lower cost devices can be enhanced, simply and cheaply. Thus, the invention enables the use of smaller (and cheaper) magnetic sources, such as molded magnets, for any desired flux density at nearly maximum efficiencies, and provided for achieving remarkably high usable flux densities in magnetic and electromagnetic devices.
From the viewpoint of magnetic circuit design, use of magnetic circuits according to the invention affords the assumption of zero flux loss due to leakage from the magnetically conducting volume. No loss factor, therefore, need be accounted for or determined.